Led array between flexible and rigid substrates

ABSTRACT

A light source can include a transparent flexible substrate. A sparse array of light-emitting diodes (LEDs) can be disposed on the transparent flexible substrate. A rigid substrate can be adhered to the transparent flexible substrate such that the sparse array of LEDs is located between the rigid substrate and the transparent flexible substrate. The rigid substrate can be adhered to the transparent flexible substrate with an adhesive layer. The sparse array of LEDs can be encapsulated in an adhesive of the adhesive layer. The adhesive can have a refractive index that is between a refractive index of the transparent flexible substrate and a refractive index of the rigid substrate, inclusive. The sparse array of LEDs can have a light-emitting area that is less than or equal to a specified fraction of a surface area of the sparse array, such as 5 percent or 1 percent.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 63/352,517, filed Jun. 15, 2022, which is incorporated by reference herein in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to sparse arrays of light-emitting diodes (LEDs).

BACKGROUND OF THE DISCLOSURE

There is ongoing effort to improve technology that uses sparse arrays of LEDs, such as micro-LEDs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional side view of an example of a light source.

FIG. 2 shows a block diagram of an example of a visualization system, which can include the light source of FIG. 1 .

FIGS. 3 and 4 show side-view drawings of the light source of FIG. 1 in various stages of assembly.

FIG. 5 shows a flowchart of an example of a method for fabricating a light source.

Corresponding reference characters indicate corresponding parts throughout the several views. Elements in the drawings are not necessarily drawn to scale. The configurations shown in the drawings are merely examples and should not be construed as limiting in any manner.

DETAILED DESCRIPTION

For the purposes of this document, the term “micro-LED” is intended to be synonymous with an LED of a sparse array of LEDs. There is ongoing effort to improve micro-LED display technology. For example, displays, such as direct-view displays and projection displays, can use micro-LEDs to improve efficiency and increase brightness.

In a direct-view micro-LED display, the LEDs may occupy a relatively small fraction of the display area. Because most of the display area is unaffected by the LEDs, the LEDs may not substantially alter the optical properties of the surface on which they are assembled. For example, a black surface may remain black in the presence of LEDs mounted on the black surface. Similarly, a reflective surface may remain reflective in the presence of LEDs mounted on the reflective surface. Other examples and optical surface properties can also be used.

In some examples, the micro-LEDs can be assembled onto a transparent flexible substrate. The transparent flexible substrate can then be laminated onto a substrate that has desired optical properties, such as being reflective, and so forth. Using the transparent flexible substrate in this manner can allow micro-LEDs to be applied to a curved or irregularly shaped substrate, which may not be compatible with micro-LED assembly technologies that use a rigid, flat substrate, such as a wafer.

In some examples, the flexible substrate can be laminated LED-side down onto the substrate, using a transparent adhesive that has sufficient thickness to encapsulate the micro-LEDs. For these examples and others, the transparent substrate and adhesive can also function as a barrier that can protect the micro-LEDs from the environment. Because the transparent substrate can provide protection for the micro-LEDs, the transparent substrate can reduce or eliminate the need to use an additional transparent cover or protection layer to provide the protection for the micro-LEDs.

FIG. 1 shows a cross-sectional side view of an example of a light source 100. The light source 100 can include a sparse array of LEDs 102 (e.g., “micro-LEDs”) disposed on a transparent flexible substrate 104, and a rigid substrate 106 adhered to the transparent flexible substrate 104 with an adhesive layer 108 such that the sparse array of LEDs 102 is located between the rigid substrate 106 and the transparent flexible substrate 104. The sparse array of LEDs 102 can be encapsulated in the adhesive 110 of the adhesive layer 108.

The transparent flexible substrate 104 can be a polymer sheet with a relatively high transmittance, or, equivalently, relatively low losses due to absorption and scattering in the visible portion of the electromagnetic spectrum, such as between wavelengths of about 400 nm and about 700 nm. Suitable materials for the transparent flexible substrate 104 can include clear polyimide (PI), polyethylene naphthalate (PEN), polyethylene terephthalate (PET) and others. The transparent flexible substrate 104 can have a thickness between about 20 μm and about 200 μm, although a thickness outside this range of thicknesses can also be used.

The sparse array of LEDs 102 can be disposed on the transparent flexible substrate 104. For the purposes of this document, the term sparse is intended to signify that a light-producing surface area of the array is less, or significantly less, than a total surface area of the array. For example, a fill factor of the array (e.g., a ratio of light-producing surface area to full surface area) can be less than or equal to a specified threshold, such as 10%, 5%, 4%, 3%, 2%, 1%, or another suitable threshold. As a specific example, the LEDs 102 can be arranged in a rectangular array, with center-to-center spacing along one dimension denoted by spacing x. Each LED 102 can have a light-producing area sized along the one dimension by size s. The ratio of s divided by x can be less than or equal to 0.1. In an orthogonal dimension, a similar ratio applies, with the linear size of a light-producing area being less than or equal to one-tenth the linear center-to-center spacing of the LEDs 102. Combining the two linear dimensions, the surface area of the light-producing areas of the LEDs 102 is less than or equal to 1% of the surface area of the array. In some examples, the light-producing area of each LED can be smaller than 200 μm on a side. In some examples, the light-producing area of each LED can be smaller than 50 μm on a side. Electrical traces can be deposited on the transparent flexible substrate 104 to electrically power (e.g., carry current to and from) the LEDs 102. In some examples, the electrical traces can be metal traces that are narrow enough to be invisible under typical viewing conditions. In some examples the electrical traces can be formed from one or more transparent electrically conductive materials, such as indium tin oxide (ITO).

In some examples, the sparse array of LEDs 102 can include two or more LEDs 102 that emit light at a same wavelength (or color). In some examples, the sparse array of LEDs 102 can include LEDs 102 that all emit light at a same wavelength. For example, a device, such as a display, can include a sparse array of LEDs 102 that all emit red light, a sparse array of LEDs 102 that all emit green light, and a sparse array of LEDs 102 that all emit blue light. In some examples, the sparse array of LEDs 102 can include two or more LEDs 102 that emit light at different wavelengths. For example, a device, such as a display, can include a sparse array of LEDs 102 in which some LEDs 102 emit red light, some LEDs 102 emit green light, and some LEDs 102 can emit blue light. The red, green, and blue LEDs 102 can be arranged in repeating clusters, with each cluster forming a color pixel of the device. In some examples, the sparse array of LEDs 102 can include at least one LED that emits light at a visible wavelength (e.g., between about 400 nm and about 700 nm). In some examples, the sparse array of LEDs 102 can include at least one LED that emits light at an infrared wavelength (e.g., greater than about 700 nm). Such infrared wavelengths can be used for biometric sensing or other sensing techniques.

Because the sparse array of LEDs 102, including the light-emitting area of the LEDs 102, the corresponding electrical traces, and any corresponding circuitry, can have a relatively small fill factor, most of the surface area of the sparse array of LEDs 102 can be transparent. For example, light incident on the sparse array of LEDs 102, either incident from the transparent flexible substrate 104 or incident on the transparent flexible substrate 104, mostly passes through the sparse array of LEDs 102, with only a relatively small fraction being blocked by the light-emitting areas and electrical traces of the sparse array of LEDs 102.

As a result, the sparse array of LEDs 102 can produce light on a surface and/or an optical element that has an additional function, such as on the rigid substrate 106, described below. For example, the surface and/or optical element can include a reflector that has a specified value of reflectance. As another example, the surface and/or optical element can include a spectral filter that has a specified reflectance, transmittance, or absorptance at one or more specified wavelengths. Other suitable functions can also be used.

The rigid substrate 106 can be adhered to the transparent flexible substrate 104.

In some examples, the rigid substrate 106 can be transparent. Suitable applications for a transparent rigid substrate 106 can include a vehicle windshield, a building window, a heads-up display, an augmented reality headset, and others. Suitable transparent materials for a transparent rigid substrate 106 can include glass, laminated glass, polycarbonate, or an engineering plastic such as poly(methyl methacrylate) (PMMA).

In some examples, the rigid substrate 106 can be reflective. In some examples, the rigid surface can be specularly reflective (e.g., can have a relatively smooth reflective surface that causes relatively little scattering or diffusion upon reflection). Suitable applications for a reflective rigid substrate 106 can include a mirror, such as a vehicular rear-view mirror or side-view mirror that can display information. Specifically, the specularly reflective surface of the rigid substrate 106 can perform the function of reflecting light from the rear of the vehicle, while the sparse array of LEDs 102 can display information superimposed on the reflected light.

In some examples, the rigid substrate 106 can be protective and/or decorative, such as a case material of a mobile device, such as a smart phone. The rigid substrate 106 can include other suitable optical properties and perform other suitable functions as well.

The rigid substrate 106 can be flat (e.g., substantially flat) or curved. Curved substrates can be used in vehicle windshields, augmented reality headsets, wearables, or other suitable devices. In some examples, the rigid substrate 106 is formed as a single unitary body. In other examples, the rigid substrate 106 can include multiple rigid substrate elements. For examples, multiple rigid substrate elements can be used to create a folding display in a smartphone or other mobile device. Custom tooling can support such curved substrates in the lamination process, described below.

The adhesive can adhere the rigid substrate 106 to the transparent flexible substrate 104. In some examples, the adhesive can be formed as an adhesive layer 108 such that the sparse array of LEDs 102 is located between the rigid substrate 106 and the transparent flexible substrate 104. Other suitable configurations can also be used. Suitable materials for the adhesive of the adhesive layer 108 can include silicone, epoxy silicone, an acrylic film, an epoxy film, and others.

In some examples, the adhesive of the adhesive layer 108 can be formed from a material having a refractive index that can match or substantially match a refractive index of the transparent flexible substrate 104 and/or the rigid substrate 106 or can fall between refractive indices of the transparent flexible substrate 104 and the rigid substrate 106. Selecting a refractive index in this manner can reduce or eliminate reflections at the interface between the adhesive layer 108 and the transparent flexible substrate 104 and/or at the interface between the adhesive layer 108 and the rigid substrate 106. For example, the adhesive of the adhesive layer 108 can be formed from a material having a refractive index between about 1.4 and about 1.7. Using a refractive index in the range of about 1.4 to about 1.7 can reduce unwanted reflections between the adhesive layer 108 and the transparent flexible substrate 104 and unwanted reflections between the adhesive layer 108 and the rigid substrate 106. Optional thin-film anti-reflection coatings can also be used to help reduce or eliminate unwanted reflections at one or more interfaces between adjacent differing materials or between a material and air.

In some examples, the adhesive layer 108 can fully encapsulate the sparse array of LEDs 102. By fully encapsulating the sparse array of LEDs 102, the adhesive layer 108 can protect the sparse array of LEDs 102 from the environment and can form a smooth, unbroken interface with the rigid substrate 106. To fully encapsulate the sparse array of LEDs 102, the adhesive of the adhesive layer 108 can have a resin viscosity that is low enough such that the adhesive flows around the LEDs 102 as the adhesive is deposited. In addition, to fully encapsulate the sparse array of LEDs 102, the adhesive layer 108 can be thick enough to fully cover the topography of the sparse array of LEDs 102.

FIG. 2 shows a block diagram of an example of a visualization system 10, which can include the light source 100 of FIG. 1 . The visualization system 10 can include a wearable housing 12, such as a headset or goggles. The housing 12 can mechanically support and house the elements detailed below. In some examples, one or more of the elements detailed below can be included in one or more additional housings that can be separate from the wearable housing 12 and couplable to the wearable housing 12 wirelessly and/or via a wired connection. For example, a separate housing can reduce the weight of wearable goggles, such as by including batteries, radios, and other elements. The housing 12 can include one or more batteries 14, which can electrically power any or all of the elements detailed below. The housing 12 can include circuitry that can electrically couple to an external power supply, such as a wall outlet, to recharge the batteries 14. The housing 12 can include one or more radios 16 to communicate wirelessly with a server or network via a suitable protocol, such as WiFi.

The visualization system 10 can include one or more sensors 18, such as optical sensors, audio sensors, tactile sensors, thermal sensors, gyroscopic sensors, time-of-flight sensors, triangulation-based sensors, and others. In some examples, one or more of the sensors can sense a location, a position, and/or an orientation of a user. In some examples, one or more of the sensors 18 can produce a sensor signal in response to the sensed location, position, and/or orientation. The sensor signal can include sensor data that corresponds to a sensed location, position, and/or orientation. For example, the sensor data can include a depth map of the surroundings. In some examples, such as for an augmented reality system, one or more of the sensors 18 can capture a real-time video image of the surroundings proximate a user.

The visualization system 10 can include one or more video generation processors 20. The one or more video generation processors 20 can receive, from a server and/or a storage medium, scene data that represents a three-dimensional scene, such as a set of position coordinates for objects in the scene or a depth map of the scene. The one or more video generation processors 20 can receive one or more sensor signals from the one or more sensors 18. In response to the scene data, which represents the surroundings, and at least one sensor signal, which represents the location and/or orientation of the user with respect to the surroundings, the one or more video generation processors 20 can generate at least one video signal that corresponds to a view of the scene. In some examples, the one or more video generation processors 20 can generate two video signals, one for each eye of the user, that represent a view of the scene from a point of view of the left eye and the right eye of the user, respectively. In some examples, the one or more video generation processors 20 can generate more than two video signals and combine the video signals to provide one video signal for both eyes, two video signals for the two eyes, or other combinations.

The visualization system 10 can include one or more light sources 22 (such as the light source 100 of FIG. 1 ) that can provide light for a display of the visualization system 10. Suitable light sources 22 can include a light-emitting diode, a monolithic light-emitting diode, a plurality of light-emitting diodes, an array of light-emitting diodes, an array of light-emitting diodes disposed on a common substrate, a segmented light-emitting diode that is disposed on a single substrate and has light-emitting diode elements that are individually addressable and controllable (and/or controllable in groups and/or subsets), an array of micro-light-emitting diodes (microLEDs), and others. In some examples, one or more of the light sources 22 can include a sparse array of LEDs disposed on a transparent flexible substrate, and a rigid substrate adhered to the transparent flexible substrate with an adhesive layer such that the sparse array of LEDs is located between the rigid substrate and the transparent flexible substrate.

A light-emitting diode can be white-light light-emitting diode. For example, a white-light light-emitting diode can emit excitation light, such as blue light or violet light. The white-light light-emitting diode can include one or more phosphors that can absorb some or all of the excitation light and can, in response, emit phosphor light, such as yellow light, that has a wavelength greater than a wavelength of the excitation light.

The one or more light sources 22 can include light-producing elements having different colors or wavelengths. For example, a light source can include a red light-emitting diode that can emit red light, a green light-emitting diode that can emit green light, and a blue light-emitting diode that can emit blue right. The red, green, and blue light combine in specified ratios to produce any suitable color that is visually perceptible in a visible portion of the electromagnetic spectrum.

The visualization system 10 can include one or more modulators 24. The modulators 24 can be implemented in one of at least two configurations.

In a first configuration, the modulators 24 can include circuitry that can modulate the light sources 22 directly. For example, the light sources 22 can include an array of light-emitting diodes, and the modulators 24 can directly modulate the electrical power, electrical voltage, and/or electrical current directed to each light-emitting diode in the array to form modulated light. The modulation can be performed in an analog manner and/or a digital manner. In some examples, the light sources 22 can include an array of red light-emitting diodes, an array of green light-emitting diodes, and an array of blue light-emitting diodes, and the modulators 24 can directly modulate the red light-emitting diodes, the green light-emitting diodes, and the blue light-emitting diodes to form the modulated light to produce a specified image.

In a second configuration, the modulators 24 can include a modulation panel, such as a liquid crystal panel. The light sources 22 can produce uniform illumination, or nearly uniform illumination, to illuminate the modulation panel. The modulation panel can include pixels. Each pixel can selectively attenuate a respective portion of the modulation panel area in response to an electrical modulation signal to form the modulated light. In some examples, the modulators 24 can include multiple modulation panels that can modulate different colors of light. For example, the modulators 24 can include a red modulation panel that can attenuate red light from a red light source such as a red light-emitting diode, a green modulation panel that can attenuate green light from a green light source such as a green light-emitting diode, and a blue modulation panel that can attenuate blue light from a blue light source such as a blue light-emitting diode.

In some examples of the second configuration, the modulators 24 can receive uniform white light or nearly uniform white light from a white light source, such as a white-light light-emitting diode. The modulation panel can include wavelength-selective filters on each pixel of the modulation panel. The panel pixels can be arranged in groups (such as groups of three or four), where each group can form a pixel of a color image. For example, each group can include a panel pixel with a red color filter, a panel pixel with a green color filter, and a panel pixel with a blue color filter. Other suitable configurations can also be used.

The visualization system 10 can include one or more modulation processors 26, which can receive a video signal, such as from the one or more video generation processors 20, and, in response, can produce an electrical modulation signal. For configurations in which the modulators 24 directly modulate the light sources 22, the electrical modulation signal can drive the light sources 24. For configurations in which the modulators 24 include a modulation panel, the electrical modulation signal can drive the modulation panel.

The visualization system 10 can include one or more beam combiners 28 (also known as beam splitters 28), which can combine light beams of different colors to form a single multi-color beam. For configurations in which the light sources 22 can include multiple light-emitting diodes of different colors, the visualization system 10 can include one or more wavelength-sensitive (e.g., dichroic) beam splitters 28 that can combine the light of different colors to form a single multi-color beam.

The visualization system 10 can direct the modulated light toward the eyes of the viewer in one of at least two configurations. In a first configuration, the visualization system 10 can function as a projector, and can include suitable projection optics 30 that can project the modulated light onto one or more screens 32. The screens 32 can be located a suitable distance from an eye of the user. The visualization system 10 can optionally include one or more lenses 34 that can locate a virtual image of a screen 32 at a suitable distance from the eye, such as a close-focus distance, such as 500 mm, 750 mm, or another suitable distance. In some examples, the visualization system 10 can include a single screen 32, such that the modulated light can be directed toward both eyes of the user. In some examples, the visualization system 10 can include two screens 32, such that the modulated light from each screen 32 can be directed toward a respective eye of the user. In some examples, the visualization system 10 can include more than two screens 32. In a second configuration, the visualization system 10 can direct the modulated light directly into one or both eyes of a viewer. For example, the projection optics 30 can form an image on a retina of an eye of the user, or an image on each retina of the two eyes of the user.

For some configurations of augmented reality systems, the visualization system 10 can include an at least partially transparent display, such that a user can view the user's surroundings through the display. For such configurations, the augmented reality system can produce modulated light that corresponds to the augmentation of the surroundings, rather than the surroundings itself. For example, in the example of a retailer showing a chair, the augmented reality system can direct modulated light, corresponding to the chair but not the rest of the room, toward a screen or toward an eye of a user.

FIGS. 3 and 4 show side-view drawings of the light source 100 of FIG. 1 in various stages of assembly. Assembling the light source 100 in this manner can help avoid air occlusions, which can degrade the appearance and optical performance of the finished device. Other assembly techniques can also be used.

In FIG. 3 , a sparse array of LEDs 102 has been disposed (e.g., mounted, grown, deposited, or otherwise formed) on the transparent flexible substrate 104. The transparent flexible substrate 104 has been removably mounted in or on a frame 302. Lamination tooling 304 can advance the frame 302 toward an adhesive layer 306 that has been deposited on a support film 308. Alternatively, the lamination tooling 304 can advance the adhesive layer 306 toward the frame 302, or advance both toward each other. The sparse array of LEDs 102 will contact the adhesive layer 306, which has been deposited on the support film 308. The adhesive of the adhesive layer 306 can be in solid form at room temperature. The adhesive has been coated on the support film 308 and has been covered by a cover film (not shown). A vacuum laminator with a temperature range that allows the adhesive to flow and conformally cover the LED array topography can laminate the adhesive layer 306 onto the sparse array of LEDs 102 to encapsulate the sparse array of LEDs 102 in the adhesive layer 108. After lamination, the support film 308 can then be removed.

In FIG. 4 , the lamination tooling 404 (optionally the same lamination tooling 304 as in FIG. 3 ) can advance the transparent flexible substrate 104 and the adhesive layer 108, with the sparse array of LEDs 102 encapsulated in the adhesive layer 108, toward the rigid substrate 106. Alternatively, the lamination tooling 404 can advance the rigid substrate 106 toward the transparent flexible substrate 104 and the adhesive layer 108, or advance both toward each other. The lamination tooling can laminate the adhesive layer 108, with the sparse array of LEDs 102 encapsulated in the adhesive layer 108, onto the rigid substrate 106. After the adhesive layer 108 has been laminated onto the rigid substrate 106, the full layered structure can be subjected to a temperature cycle to cure the adhesive.

In the examples described above, the transparent flexible substrate 104 has been laminated with the LED side facing the rigid substrate 106, such that the LEDs 102 are located between the two substrates and encapsulated by the adhesive. Alternatively, the transparent flexible substrate 104 can be laminated with the LED side facing away from the rigid substrate 106, such that the LEDs 102 are exposed, and the transparent flexible substrate 104 is located between the exposed LEDs 102 and the rigid substrate 106.

In some examples, additional components can be assembled onto the transparent flexible substrate, such as integrated circuits (ICs), micro-ICs, or transistors for display backplanes. Moreover, additional layers may be integrated into a device to form a capacitive or resistive touchscreen.

FIG. 5 shows a flowchart of an example of a method 500 for fabricating a light source. The method 500 can be used to fabricate the light source 100 of FIG. 1 , or any other suitable light source. Other methods can also be used to fabricate the light source 100 of FIG. 1 .

At operation 502, a transparent flexible substrate is provided.

At operation 504, a sparse array of light-emitting diodes (LEDs) is disposed on the transparent flexible substrate.

At operation 506, a rigid substrate is adhered to the transparent flexible substrate such that the sparse array of LEDs is located between the rigid substrate and the transparent flexible substrate.

To further illustrate the systems and related methods disclosed herein, a non-limiting list of examples is provided below. Each of the following non-limiting examples can stand on its own or can be combined in any permutation or combination with any one or more of the other examples.

In Examples 1, a light source can comprise: a transparent flexible substrate; a sparse array of light-emitting diodes (LEDs) disposed on the transparent flexible substrate; and a rigid substrate adhered to the transparent flexible substrate such that the sparse array of LEDs is located between the rigid substrate and the transparent flexible substrate.

In Example 2, the light source of Example 1 can optionally be configured such that the rigid substrate is adhered to the transparent flexible substrate with an adhesive layer, the sparse array of LEDs being encapsulated in an adhesive of the adhesive layer.

In Example 3, the light source of any one of Examples 1-2 can optionally be configured such that: the rigid substrate is substantially transparent; and the adhesive has a refractive index that is between a refractive index of the transparent flexible substrate and a refractive index of the rigid substrate, inclusive.

In Example 4, the light source of any one of Examples 1-3 can optionally be configured such that the rigid substrate is substantially transparent.

In Example 5, the light source of any one of Examples 1˜4 can optionally be configured such that the sparse array of LEDs has a light-emitting area that is less than or equal to 5% of a surface area of the sparse array.

In Example 6, the light source of any one of Examples 1-5 can optionally be configured such that the sparse array of LEDs has a light-emitting area that is less than or equal to 1% of a surface area of the sparse array.

In Example 7, the light source of any one of Examples 1-6 can optionally further comprise transparent electrical traces configured to electrically power the LEDs of the sparse array of LEDs.

In Example 8, the light source of any one of Examples 1-7 can optionally be configured such that the sparse array of LEDs includes two or more LEDs that emit light at a same wavelength.

In Example 9, the light source of any one of Examples 1-8 can optionally be configured such that the sparse array of LEDs includes two or more LEDs that emit light at different wavelengths.

In Example 10, the light source of any one of Examples 1-9 can optionally be configured such that the rigid substrate is specularly reflective.

In Example 11, the light source of any one of Examples 1-10 can optionally be configured such that the rigid substrate is substantially flat.

In Example 12, the light source of any one of Examples 1-11 can optionally be configured such that the rigid substrate is curved.

In Example 13, the light source of any one of Examples 1-12 can optionally be configured such that the rigid substrate is a single unitary element.

In Example 14, the light source of any one of Examples 1-13 can optionally be configured such that the rigid substrate includes multiple rigid substrate elements.

In Example 15, a method for fabricating a light source can comprise: providing a transparent flexible substrate; disposing a sparse array of light-emitting diodes (LEDs) on the transparent flexible substrate; and adhering a rigid substrate to the transparent flexible substrate such that the sparse array of LEDs is located between the rigid substrate and the transparent flexible substrate.

In Example 16, the method of Example 15 can optionally be configured such that the rigid substrate is adhered to the transparent flexible substrate with an adhesive layer, the sparse array of LEDs being encapsulated in an adhesive of the adhesive layer.

In Example 17, the method of any one of Examples 15-16 can optionally be configured such that the adhesive has a refractive index that is between a refractive index of the transparent flexible substrate and a refractive index of the rigid substrate, inclusive.

In Example 18, a light source can comprise: a transparent flexible substrate; a sparse array of light-emitting diodes (LEDs) disposed on the transparent flexible substrate, the sparse array of LEDs having a light-emitting area that is less than or equal to 10% of a surface area of the sparse array; and a rigid substrate adhered with an adhesive layer to the transparent flexible substrate such that the sparse array of LEDs is encapsulated in an adhesive of the adhesive layer.

In Example 19, the light source of Example 18 can optionally be configured such that the sparse array of LEDs includes two or more LEDs that emit light at a same wavelength.

In Example 20, the light source of any one of Examples 18-19 can optionally be configured such that sparse array of LEDs includes two or more LEDs that emit light at different wavelengths.

While only certain features of the system and method have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes. Method operations can be performed substantially simultaneously or in a different order. 

What is claimed is:
 1. A light source, comprising: a transparent flexible substrate; a sparse array of light-emitting diodes (LEDs) disposed on the transparent flexible substrate; and a rigid substrate adhered to the transparent flexible substrate such that the sparse array of LEDs is located between the rigid substrate and the transparent flexible substrate.
 2. The light source of claim 1, wherein the rigid substrate is adhered to the transparent flexible substrate with an adhesive layer, the sparse array of LEDs being encapsulated in an adhesive of the adhesive layer.
 3. The light source of claim 2, wherein: the rigid substrate is substantially transparent; and the adhesive has a refractive index that is between a refractive index of the transparent flexible substrate and a refractive index of the rigid substrate, inclusive.
 4. The light source of claim 1, wherein the rigid substrate is substantially transparent.
 5. The light source of claim 1, wherein the sparse array of LEDs has a light-emitting area that is less than or equal to 5% of a surface area of the sparse array.
 6. The light source of claim 1, wherein the sparse array of LEDs has a light-emitting area that is less than or equal to 1% of a surface area of the sparse array.
 7. The light source of claim 1, further comprising transparent electrical traces configured to electrically power the LEDs of the sparse array of LEDs.
 8. The light source of claim 1, wherein the sparse array of LEDs includes two or more LEDs that emit light at a same wavelength.
 9. The light source of claim 1, wherein the sparse array of LEDs includes two or more LEDs that emit light at different wavelengths.
 10. The light source of claim 1, wherein the rigid substrate is specularly reflective.
 11. The light source of claim 1, wherein the rigid substrate is substantially flat.
 12. The light source of claim 1, wherein the rigid substrate is curved.
 13. The light source of claim 1, wherein the rigid substrate is a single unitary element.
 14. The light source of claim 1, wherein the rigid substrate includes multiple rigid substrate elements.
 15. A method for fabricating a light source, the method comprising: providing a transparent flexible substrate; disposing a sparse array of light-emitting diodes (LEDs) on the transparent flexible substrate; and adhering a rigid substrate to the transparent flexible substrate such that the sparse array of LEDs is located between the rigid substrate and the transparent flexible substrate.
 16. The method of claim 15, wherein the rigid substrate is adhered to the transparent flexible substrate with an adhesive layer, the sparse array of LEDs being encapsulated in an adhesive of the adhesive layer.
 17. The method of claim 16, wherein the adhesive has a refractive index that is between a refractive index of the transparent flexible substrate and a refractive index of the rigid substrate, inclusive.
 18. A light source, comprising: a transparent flexible substrate; a sparse array of light-emitting diodes (LEDs) disposed on the transparent flexible substrate, the sparse array of LEDs having a light-emitting area that is less than or equal to 10% of a surface area of the sparse array; and a rigid substrate adhered with an adhesive layer to the transparent flexible substrate such that the sparse array of LEDs is encapsulated in an adhesive of the adhesive layer.
 19. The light source of claim 18, wherein the sparse array of LEDs includes two or more LEDs that emit light at a same wavelength.
 20. The light source of claim 18, wherein the sparse array of LEDs includes two or more LEDs that emit light at different wavelengths. 